quickselect

最新推荐文章于 2025-03-15 16:45:34 发布

原创最新推荐文章于 2025-03-15 16:45:34 发布 · 3.6k 阅读

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文章标签：

#list #pivot #function

本文深入解析了快速选择算法的核心逻辑，包括分区函数、选择函数及其优化策略，详细阐述了算法的工作原理、实现细节以及实际应用。

function
 partition(list, left, right, pivotIndex)
     pivotValue := list[pivotIndex]
     swap list[pivotIndex] and list[right]  // Move pivot to end

     storeIndex := left
     for
 i from
 left to
 right
         if
 list[i] < pivotValue
             swap list[storeIndex] and list[i]
             increment storeIndex
     swap list[right] and list[storeIndex]  // Move pivot to its final place

     return
 storeIndex





function
 select(list, left, right, k)
     if
 left = right // If the list contains only one element
         return
 list[left]  // Return that element
     select pivotIndex between left and right
     pivotNewIndex := partition(list, left, right, pivotIndex)
     pivotDist := pivotNewIndex - left + 1 
     // The pivot is in its final sorted position, 
     // so pivotDist reflects its 1-based position if list were sorted
     if
 pivotDist = k 
         return
 list[pivotNewIndex]
     else if
 k < pivotDist 
         return
 select(list, left, pivotNewIndex - 1, k)
     else

         return
 select(list, pivotNewIndex + 1, right, k - pivotDist)



function
 select(list, left, right, k)
     loop

         select pivotIndex between left and right
         pivotNewIndex := partition(list, left, right, pivotIndex)
         pivotDist := pivotNewIndex - left + 1
         if
 pivotDist = k
             return
 list[pivotNewIndex]
         else if
 k < pivotDist
             right := pivotNewIndex - 1
         else

             k := k - pivotDist
             left := pivotNewIndex + 1